The present invention concerns a process for removing sulfur dioxide and nitrogen oxides from stack gases containing the same which comprises contacting such gases at temperatures in excess of about 700.degree. C and sufficient to remove in general greater than 80 percent of the sulfur dioxide and nitrogen oxides present with a solid reagent comprising calcium oxide, magnesium oxide, or mixtures thereof in the presence of a reducing agent which comprises carbon monoxide, hydrogen, or mixtures thereof.
The term "nitrogen oxides" as employed herein encompasses any one or more of five different nitrogen oxides--nitric oxide (NO), nitrogen dioxide (NO.sub.2), dinitrogen oxide (N.sub.2 O), dinitrogen trioxide (N.sub.2 O.sub.3), and dinitrogen pentoxide (N.sub.2 O.sub.5)--although of this group only nitric oxide and nitrogen dioxide are known to play a significant role in air quality problems, and it is these two nitrogen oxides with which the present process is particularly concerned. The nitrogen oxides as found in stack gases and other combustion products are represented in the current literature by the general formula "NO.sub.(x)."
The process of the present invention is characterized by maintaining, in the presence of the reducing agent, an excess of the solid reagent sufficient to facilitate the efficient reaction of sulfur dioxide under the reaction conditions. The maintenance of the excess of solid reagent makes possible under the reaction conditions employed the utilization of a high space velocity for the stack gases.
The process is further characterized by effective and efficient removal of sulfur dioxide and nitrogen oxides--in general greater than 80 percent removal--at the high space velocities herein employed.
The term "space velocity" as employed herein means the reciprocal of the contact time of the stack gases with the solid reagent.
The present process, in addition to providing for effective and efficient removal of sulfur dioxide and nitrogen oxides from stack gases containing the same by conversion, respectively, into calcium or magnesium sulfide and nitrogen gas, also provides for regeneration of the original solid reagent from the corresponding sulfide product. This regenerative procedure facilitates both the further use of the solid reagent in the process and simultaneously, the recovery of valuable sulfur values as either pure hydrogen sulfide, pure elemental sulfur, or high grade sulfuric acid.
Sulfur dioxide and nitrogen oxides are noxious components of stack gases. And even though they are generally present in relatively low concentrations, large amounts of both sulfur dioxide and nitrogen oxides are produced each year as waste products. It has been estimated that 36.6 .times. 10.sup.6 tons of sulfur dioxide were emitted from stationary sources in 1971 and that sulfur dioxide emissions could quadruple by the year 2000 if adequate controls are not provided. Approximately one-half of this sulfur dioxide emission level is due to power plants burning coal and oil.
Thus millions of tons of sulfur are wasted each year by the discharge into the atmosphere of stack gases containing sulfur dioxide. And, not only is the discharge of sulfur dioxide into the atmosphere a waste of a valuable chemical, sulfur dioxide also has a damaging effect on human beings, animals, agricultural areas and in general is one of the major causes of air pollution.
Nitrogen oxides also occur in stack gases. These oxides are significant contributors to smog formation. In 1968 nitrogen oxides emissions were estimated to be about 16 .times. 10.sup.6 tons, about 60 percent of which originated from stationary sources.
The fact that the concentrations of both sulfur dioxide and nitrogen oxides in stack gases are low, for example, less than 0.3 percent by volume, and the fact that the concentrations are to be reduced to significantly lower values, for example, less than 0.025 percent, presents a formidable removal problem.
Several processes have been suggested for reducing sulfur dioxide and nitrogen oxides emissions from stacks of combustion operations. One such suggestion is to use fuels low in sulfur content but such fuels are generally regarded as too expensive for widespread use. Other suggestions have included reduction of sulfur or nitrogen content of fuels, modification of the combustion process, and various wet and dry processes for removing sulfur dioxide and nitrogen oxides from the stack gases. Kinetic processes for removing sulfur dioxide and nitrogen oxides by reduction in the presence of reducing gases and suitable catalysts to elemental sulfur and nitrogen gas have also been considered. Each of these processes, however, has a variety of well publicized disadvantages, including cost, by-product sales, associated water or solid waste problems, and a general lack of efficiency. In addition, during each of the suggested processes for noxious gas abatement significant cooling of the gases must be avoided. Otherwise, the cooled gases lose their buoyancy and descend in the neighborhood of the stack. This results in a greater local awareness of the presence of sulfur dioxide and nitrogen oxides than before the attempted removal was initiated unless essentially complete removal of the noxious gases is effected.
It is therefore an object of the present invention to provide an effective and efficient process for removing sulfur dioxide and nitrogen oxides from stack gases.
Various other objects and advantages of this invention will become apparent from the accompanying description and disclosure.
The above and other objects have been accomplished in a very effective and efficient manner by the process of the present invention. The unexpected success with which the objects have been accomplished is apparent from the unique properties and advantages of the present process as described in the following specification. Briefly, however, these include:
a. Simultaneous removal of sulfur dioxide and nitrogen oxides from stack gases containing the same; PA1 b. The enhancement of the removal of nitrogen oxides by the initially added solid reagent as well as the corresponding sulfide produced therefrom in the presence of sulfur dioxide without any increase in production of undesirable side products such as hydrogen sulfide and carbon oxysulfide; PA1 c. The absence of any further reduction of the corresponding sulfide produced from the solid reagent to undesirable side products under the reaction conditions employed in the process; PA1 d. The necessity of only providing an excess of reducing agent without any requirement to provide a stoichiometric amount; PA1 e. The efficiency and effectiveness of the present process at high space velocities and low concentrations; PA1 f. Easy regeneration of the solid reagent for further use in the present process. PA1 g. Production of high grade sulfur by-products; and PA1 h. Flexibility of choice in by-product.